Switching power supply

ABSTRACT

A switching power supply which can prevent the output voltage Vout from fluctuating or undershooting when the instruction for stopping the operation is issued is disclosed. A switching power supply according to the present invention employs a transformer having a primary coil and a secondary coil, a switching circuit connected between an input terminal and the primary coil of the transformer, a rectifier connected to the secondary coil of the transformer, a smoothing circuit located at a subsequent stage of the rectifier and including an output capacitor, a control circuit controlling the switching circuit, and an operating voltage generating circuit, responsive to an instruction for stopping a switching operation, for supplying an operating voltage to the control circuit using at least energy stored in the output capacitor. Thus, the switching power supply of the present invention can substantially linearly lower the output voltage Vout.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a switching power supply, andmore specifically, to a switching power supply that can prevent anoutput voltage Vout from undershooting and fluctuating when theoperation of the switching power supply is stopped.

DESCRIPTION OF THE PRIOR ART

[0002] Switching power supplies are widely used as power supplies forelectrical and electronic equipment such as computers.

[0003]FIG. 7 is a circuit diagram showing a conventional switching powersupply.

[0004] As shown in FIG. 7, the conventional switching power supply iscomposed of a transformer T1, a switching circuit located on the primaryside of the transformer T1, and a rectifier of the self-drive type and asmoothing circuit located on the secondary side of the transformer T1.The switching power supply lowers a DC (direct current) input voltageVin supplied to the switching circuit located on the primary side togenerate a DC output voltage Vout and supplies it to a load. In FIG. 7,the load is represented by a resistance component RLoad, capacitancecomponent CLoad, and reactance component LLoad.

[0005] A control circuit 10 controls main switches Q1 and Q2 included inthe switching circuit of the primary side based on the output voltageVout. Specifically, the control circuit 10 lowers the duty factor of themain switches Q1 and Q2 when the output voltage Vout increases relativeto the desired voltage so as to decrease the electric power supplied tothe load and raises the duty factor of the main switches Q1 and Q2 whenthe output voltage Vout decreases relative to the desired voltage so asto increase the electric power supplied to the load. Thus, the outputvoltage Vout supplied to the load can be always stabilized at thedesired voltage. Because the control circuit 10 belongs to the primaryside, the control circuit 10 cannot receive the output voltage Voutdirectly. The control circuit 10 is therefore supplied via an isolationcircuit 20 with a voltage Vout′ associated with the output voltage Vout.

[0006] Operating voltage Vcc for the control circuit 10 is generated byan operating voltage generation circuit consisting of a transistor Tr1,resistor R1, and zener diode Z1. A capacitor C3 is connected betweenpower terminals of the control circuit 10 for stabilizing the operatingvoltage Vcc. The operating voltage generation circuit is activated whenan operation switch S1 is in the ON state and inactivated when theoperation switch S1 is in the OFF state. The operation switch S1 can becontrolled from the outside. When the operation of the switching powersupply shown in FIG. 7 is to be started, the operation switch S1 isturned ON; when the operation of the switching power supply is to beterminated, the operation switch S1 is turned OFF.

[0007] Rectifying switches Q3 and Q4 included in the rectifier of thesecondary side are self-driven by the secondary voltage of thetransformer T1. Further, resistors R2 and R3 are inserted between thegate electrodes and the source electrodes of the rectifying switches Q3and Q4, respectively, so as to prevent the gate electrodes of therectifying switches Q3 and Q4 from being in a floating state.

[0008] Next, the operation of the conventional switching power supplyshown in FIG. 7 will be explained.

[0009]FIG. 8 is a timing chart showing the operation of the conventionalswitching power supply shown in FIG. 7.

[0010] As shown in FIG. 8, when the operation switch S1 is in the ONstate, the gate-source voltages V_(GS)(Q1) and V_(GS)(Q2) of the mainswitches Q1 and Q2 are alternately activated to a high level at apredetermined switching frequency under the control of the controlcircuit 10. As a result, the polarity of the primary voltage V_(LP) ofthe transformer T1 is alternately inversed, so that primary sidecapacitors C1 and C2 are alternately charged and discharged.

[0011] Synchronously with the operation of the primary side, thepolarity of the secondary voltage appearing at secondary coils Ls1 andLs2 of the transformer T1 is alternately inversed, so that therectifying switches Q3 and Q4 are alternately brought into ON state inturn at the predetermined switching frequency. More specifically, whilethe main switch Q1 is in the ON state owing to the gate-source voltageV_(GS)(Q1) being at a high level, the gate-source voltage V_(GS)(Q3) ofthe rectifying switch Q3 is raised to a voltage greater than thethreshold voltage thereof by the voltage (secondary voltage) appearingat secondary coil Ls2, whereby the rectifying switch Q3 turns ON. On thecontrary, while the main switch Q2 is in the ON state owing to thegate-source voltage V_(GS)(Q2) being at a high level, the gate-sourcevoltage V_(GS)(Q4) of the rectifying switch Q4 is raised to a voltagegreater than the threshold voltage thereof by the voltage (secondaryvoltage) appearing at secondary coil Ls1, whereby the rectifying switchQ4 turns ON.

[0012] As a result, the secondary voltage of alternately inversedpolarity is rectified. The rectified voltage is smoothed by thesmoothing circuit, which consists of an output reactor Lout and outputcapacitor Cout so that the stabilized output voltage Vout is generated.

[0013] On the other hand, when the operation switch S1 is turned OFF ata certain time, the operation of the control circuit 10 is stoppedbecause the transistor Tr1 turns OFF, so that both the main switches Q1and Q2 are put in the OFF state. That is, the switching operation isstopped.

[0014] However, because the operation of the switching circuit of theprimary side is stopped when the operation switch S1 is turned OFF, oneor the other of the rectifying switches Q3 and Q4 is kept in the ONstate and a reverse current begins to flow from the output capacitorCout and the capacitance component CLoad of the load to the outputreactor Lout.

[0015]FIG. 8 shows the case where the rectifying switch Q3 is kept inthe ON state at first in response to the operation switch S1 beingturned OFF. In this case, because the switching circuit of the primaryside is stopped, the discharge path for the electric charge of the gateelectrode of the rectifying switch Q3 is substantially only the resistorR2. Therefore, the gate-source voltage V_(GS)(Q3) of the rectifyingswitch Q3 falls gradually owing to the current flow through the resistorR2. During this period, the reverse current flowing to the outputreactor Lout continues.

[0016] On the other hand, when the rectifying switch Q3 turns OFFbecause the gate-source voltage V_(GS)(Q3) of the rectifying switch Q3falls below the threshold voltage thereof owing to the decrease of theoutput voltage Vout and the secondary voltage by discharge of the outputcapacitor Cout and the capacitance component CLoad of the load anddischarge of the electric charge from the gate electrode of therectifying switch Q3 via resistor R2, a flyback voltage rises at thetransformer T1. The flyback voltage boosts an internal voltage Vp in theswitching circuit via the transformer T1 and boosts the gate-sourcevoltage V_(GS)(Q4) of the rectifying switch Q4. Therefore, therectifying switch Q4 stays ON.

[0017] As shown in FIG. 8, because the direction of the current flowingto the output reactor Lout via the rectifying switch Q4 becomes forwardtemporarily, the output capacitor Cout and the capacitance componentCLoad of the load are charged during this period, so that the outputvoltage Vout is increased.

[0018] Then, when the direction of the current flowing to the outputreactor Lout becomes reverse, the gate-source voltage V_(GS)(Q4) of therectifying switch Q4 falls gradually owing to the decrease of the outputvoltage Vout and the secondary voltage by discharge of the outputcapacitor Cout and the capacitance component CLoad of the load anddischarge of the electric charge from the gate electrode of rectifyingswitch Q4 via resistor R3. Then, when the rectifying switch Q4 turns OFFbecause the gate-source voltage V_(GS)(Q4) of the rectifying switch Q4falls below the threshold voltage thereof, the flyback voltage risesagain at the transformer T1, which boosts the internal voltage Vp in theswitching circuit via the transformer T1 and boosts the gate-sourcevoltage V_(GS)(Q3) of the rectifying switch Q3. Therefore, therectifying switch Q3 stays ON.

[0019] Such operations are periodically repeated until the outputcapacitor Cout and the capacitance component CLoad of the load areconsumed by the secondary side circuit and the resistance componentRLoad of the load. Therefore, the output voltage Vout graduallydecreases while fluctuating over very long period compared with theswitching period and, in addition, the internal voltage Vp in theswitching circuit is gradually increased.

[0020] As described above, in the conventional switching power supply,because the output voltage Vout does not decrease linearly but fallsgradually while fluctuating over very long period compared with theswitching period even if an instruction to stop the operation of theswitching power supply is issued (the switch S1 is turned OFF), somemalfunction may arise in the load. For example, the load may be designedto discriminate when the operation of the switching power supply hasstopped and perform a certain operation when the output voltage Voutfalls below a predetermined voltage. But if the output voltage Voutgradually decreases while fluctuating, discriminating whether theswitching power supply as stopped becomes difficult.

[0021] Further, in the conventional switching power supply, because theinternal voltage Vp in the switching circuit gradually increases duringtermination of operation, electric components used on the primary sidemay be damaged. In order to prevent this, components having a highwithstand voltage must be used. This increases the cost of the switchingpower supply.

[0022] Furthermore, in the conventional switching power supply, becauselarge current flows through the output reactor Lout, the secondary coilsLs1 and Ls2 of the transformer T1 and the rectifying switches Q3 and Q4during termination of operation, the reliability of the switching powersupply may be degraded because the output reactor Lout, the secondarycoils Ls1 and Ls2 of the transformer T1 and the rectifying switches Q3and Q4 release a large amount of heat.

[0023] These problems become more pronounced as the resistance componentRLoad of the load becomes larger. Therefore, in the case where theinstruction to terminate operation is issued in a light-load condition,the problems are serious. Further, because the problems become morepronounced as the capacitance component CLoad of the load becomes large,the problems are also serious when the electric power is supplied to aload having a large capacitance component CLoad.

[0024] On the other hand, although the problems are not so serious whenthe resistance component RLoad of the load is considerably small (i.e.,the load is heavy), in this case, some malfunction may arise in the loadduring the termination of operation owing to undershoot of the outputvoltage Vout. For example, when the output voltage Vout becomesnegative, parasitic diodes and so forth in an integrated circuit (IC)employed in the load may turn ON. As this causes huge current to flowthrough the IC, the IC may malfunction or be damaged.

[0025]FIG. 9 is a timing chart showing the undershoot of the outputvoltage Vout during the termination of operation.

[0026] As shown in FIG. 9, when the operation of the switching circuitof the primary side is stopped by turning OFF the operation switch S1,the current I_(RLoad) flowing through the resistance component RLoad ischanged from the output current I_(Lout) of the output reactor Lout tothe discharge current I_(Cout) of the output capacitor Cout and thevoltage V_(LLoad) rises at the reactance component LLoad of the load, sothat current continues to flow. As a result, the output voltage Voutbecomes negative, i.e., undershoot arises. Then, if the undershootvoltage reaches the forward voltage Vf of the body diodes of therectifying switches Q3 and Q4, these body diodes turn ON. As a result,current begins to flow through the LCR serial circuit consisting of therectifying switch Q3 (body diode), the secondary coil Ls1 of thetransformer T1, the output reactor Lout, and the output capacitor Coutand another LCR serial circuit consisting of the rectifying switch Q4(body diode), the secondary coil Ls2 of the transformer T1, the outputreactor Lout, and the output capacitor Cout. Therefore, the peak valueof the undershoot voltage is clamped to about Vf.

[0027] Here, when the relationship between the resistance componentRLoad, the reactance component LLoad, and the output capacitor Coutsatisfies the formula (1), these LCR serial circuits oscillate.Undershoot arises as a result. $\begin{matrix}{{{RLoad}\quad}^{2} < {4 \cdot \frac{LLoad}{Cout}}} & (1)\end{matrix}$

[0028] As can be seen from the formula (1), undershoot tends to arisewhen the resistance component Rload is small (when the load is heavy).In order to prevent the switching power supply from undershooting, anadditional capacitor Cex of sufficient capacitance needs to be connectedin parallel with the output capacitor Cout because the resistancecomponent RLoad and the reactance component LLoad belong to the load.This leads to an undesirable increase in number of components. Thecapacitance required by the additional capacitor Cex for preventingundershoot can be represented by formula (2): $\begin{matrix}{C_{EX} > {{4 \cdot \frac{LLoad}{{{RLoad}\quad}^{2}}} - {Cout}}} & (2)\end{matrix}$

[0029] Because this problem is pronounced when the resistance componentRload is small, it becomes serious when the switching power supply isused to drive a load requiring a low voltage and a large current, suchas a server computer.

[0030] As explained above, the conventional switching power supply hastwo main problems: one is that the output voltage Vout falls graduallywhile fluctuating over a very long period when an instruction forstopping the operation of the switching power supply is issued; and theother is that undershoot arises in the output voltage Vout when theinstruction for stopping the operation of the switching power supply isissued. The former problem becomes pronounced when the resistancecomponent Rload is large, while the latter problem becomes pronouncedwhen the resistance component Rload is small. The latter problem ariseswhether or not the rectifier is a self-drive type.

SUMMARY OF THE INVENTION

[0031] It is therefore an object of the present invention to provide aswitching power supply that prevents the output voltage Vout fromundershooting when an instruction for stopping the operation of theswitching power supply is issued.

[0032] Another object of the present invention is to provide a switchingpower supply that prevents the output voltage Vout from fluctuating whenan instruction for stopping the operation of the switching power supplyis issued.

[0033] A further object of the present invention is to provide aswitching power supply that prevents the internal voltage Vp of theswitching circuit from gradually increasing when an instruction forstopping the operation of the switching power supply is issued.

[0034] A still further object of the present invention is to provide aswitching power supply that prevents a large amount of current fromflowing through the output reactor Lout, the secondary coils Ls1 and Ls2of the transformer T1 and the rectifying switches Q3 and Q4 when aninstruction for stopping the operation of the switching power supply isissued.

[0035] The above and other objects of the present invention can beaccomplished by a switching power supply, comprising:

[0036] a transformer having a primary coil and a secondary coil;

[0037] a switching circuit connected between an input terminal and theprimary coil of the transformer;

[0038] a rectifier connected to the secondary coil of the transformer;

[0039] a control circuit controlling the switching circuit; and

[0040] first and second operating voltage generating circuits eachgenerating an operating voltage of the control circuit;

[0041] a first operating voltage generated by the first operatingvoltage generating circuit and a second operating voltage generated bythe second operating voltage generating circuit having different valuesfrom each other.

[0042] In a preferred aspect of the present invention, the firstoperating voltage generating circuit includes a first zener diode whichdetermines a value of the first operating voltage and the secondoperating voltage generating circuit includes a second zener diode whichdetermines a value of the second operating voltage, a zener voltage ofthe first zener diode and a zener voltage of the second zener diodehaving different values from each other.

[0043] In a further preferred aspect of the present invention, the firstoperating voltage generating circuit further includes an operationswitch connected in parallel with the first zener diode.

[0044] In a further preferred aspect of the present invention, theswitching circuit includes first and second converters connected inseries between the input terminal and the primary coil of thetransformer.

[0045] In a further preferred aspect of the present invention, thecontrol circuit includes a first converter control circuit controllingthe first converter and a second converter control circuit controllingthe second converter, the first operating voltage generating circuitsupplying the first operating voltage to a power supply line commonlyprovided for the first and second converter control circuits, and thesecond operating voltage generating circuit supplying the secondoperating voltage to the power supply line.

[0046] In a further preferred aspect of the present invention, the firstand second converter control circuits are enabled when the firstoperating voltage generating circuit is in an active state, and thefirst converter control circuit is disabled when the second operatingvoltage generating circuit is in an active state.

[0047] In a further preferred aspect of the present invention, a minimumoperating voltage of the first converter control circuit and a minimumoperating voltage of the second converter control circuit have differentvalues from each other.

[0048] In a further preferred aspect of the present invention, theswitching power supply further comprises an auxiliary power supplycircuit for supplying a third operating voltage to the power supply lineusing a voltage appearing at an auxiliary coil provided on the primaryside of the transformer.

[0049] In a further preferred aspect of the present invention, the thirdoperating voltage is higher than the first and second operatingvoltages.

[0050] In a further preferred aspect of the present invention, the firstconverter is selected from a back converter and a boost converter, andthe first converter is selected from a half bridge converter, a forwardconverter, a full bridge converter, and a push-pull converter.

[0051] In a further preferred aspect of the present invention, therectifier is of a self-drive type.

[0052] According to these aspects of the present invention, because thefirst operating voltage generated by the first operating voltagegenerating circuit and the second operating voltage generated by thesecond operating voltage generating circuit are different, the controlcircuit can operate using the operating voltage supplied from one of theoperating voltage generating circuits during normal operation and canoperate using the operating voltage supplied from the other of theoperating voltage generating circuits after an instruction for stoppingthe operation of the switching power supply is issued. That is, theswitching operation can continue after the instruction is issued.Therefore, malfunction of the load can be effectively avoided becausethe output voltage is substantially linearly lowered without fluctuatingor undershooting.

[0053] Particularly, in the case where the rectifier is of a self-drivetype, the internal voltage in the switching circuit on the primary sideis prevented from gradually increasing when an instruction for stoppingthe operation of the switching power supply is issued The electriccomponents used on the primary side are therefore effectively protectedfrom damage. Further, because it is not necessary to use the componentshaving high withstand voltage, the cost of the switching power supplycan be lowered. Furthermore, because a large current does not flowthorough the output reactor, the secondary coil of the transformer andthe rectifier when the instruction is issued, the reliability of theswitching power supply can be enhanced.

[0054] Therefore, the switching power supply of the present invention issuitable as a switching power supply for supplying electric power to aload having a large capacitance component CLoad. Further, the switchingpower supply of the present invention is especially suitable as aswitching power supply for supplying electric power to a load that tendsto frequently assume a light-load condition. Furthermore, the switchingpower supply of the present invention is also suitable as a switchingpower supply for supplying electric power to a load that requires a lowvoltage and a large current, such as a server computer. That is,according to the present invention, the switching power supply can lowerits output voltage substantially linearly even if an instruction forstopping the operation of the switching power supply is issued atheavy-load condition or light-load condition.

[0055] The above and other objects of the present invention can be alsoaccomplished by a switching power supply, comprising:

[0056] a transformer having a primary coil and a secondary coil;

[0057] a switching circuit connected between an input terminal and theprimary coil of the transformer;

[0058] a rectifier connected to the secondary coil of the transformer;

[0059] a smoothing circuit located at a subsequent stage of therectifier and including an output capacitor;

[0060] a control circuit controlling the switching circuit; and

[0061] means, responsive to an instruction for stopping a switchingoperation, for supplying an operating voltage to the control circuitusing at least energy stored in the output capacitor.

[0062] In a preferred aspect of the present invention, the operatingvoltage supplied from the means is lower than an operating voltagesupplied to the control circuit during normal operation.

[0063] In a further preferred aspect of the present invention, therectifier is of a self-drive type.

[0064] Also according to these aspects of the present invention, theswitching power supply can lower its output voltage substantiallylinearly without fluctuating or undershooting. Malfunction of the loadcan therefore be effectively avoided. Particularly, in the case wherethe rectifier is of a self-drive type, the internal voltage in theswitching circuit of the primary side is prevented from graduallyincreasing when the instruction for stopping the operation of theswitching power supply is issued. The electric components used on theprimary side are therefore effectively protected from damage. Further,because it is not necessary to use components having high withstandvoltage, the cost of the switching power supply can be lowered.Furthermore, because a large current does not flow thorough the outputreactor, the secondary coil of the transformer and the rectifier whenthe instruction is issued, the reliability of the switching power supplycan be enhanced.

[0065] The above and other objects of the present invention can be alsoaccomplished by a switching power supply, comprising:

[0066] a transformer having a primary coil and a secondary coil;

[0067] first and second converters connected in series between an inputterminal and the primary coil of the transformer;

[0068] a rectifier connected to the secondary coil of the transformer;

[0069] a control circuit controlling the first and second converters;and

[0070] means, responsive to an instruction for stopping a switchingoperation, for stopping operations of the first and second converters inthis order.

[0071] In a preferred aspect of the present invention, the stoppingmeans causes the operation of the second converter to continue usingenergy supplied from a secondary side of the transformer during a periodfrom the time when the instruction is issued to the time of theoperation for stopping the second converter.

[0072] In a further preferred aspect of the present invention, therectifier is of a self-drive type.

[0073] Also according to these aspects of the present invention, theswitching power supply can lower its output voltage substantiallylinearly without fluctuating or undershooting. Malfunction of the loadcan therefore be effectively avoided. Particularly, in the case wherethe rectifier is of a self-drive type, the internal voltage in thesecond converter is prevented from gradually increasing when aninstruction for stopping the operation of the switching power supply isissued. The electric components used on the primary side are thereforeeffectively protected from damage. Further, because it is not necessaryto use components having high withstand voltage, the cost of theswitching power supply can be lowered. Furthermore, because a largecurrent does not flow thorough the output reactor, the secondary coil ofthe transformer and the rectifier when the instruction is issued, thereliability of the switching power supply can be enhanced.

[0074] The above and other objects and features of the present inventionwill become apparent from the following description made with referenceto the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0075]FIG. 1 is a circuit diagram showing a switching power supply thatis a preferred embodiment of the present invention.

[0076]FIG. 2 is a timing chart showing the operation of the switchingpower supply shown in FIG. 1 where a resistance component RLoad of aload is considerably large.

[0077]FIG. 3 is an enlarged timing chart showing a principal part of thetiming chart shown in FIG. 2.

[0078]FIG. 4 is a timing chart showing the operation of the switchingpower supply shown in FIG. 1 where a resistance component RLoad of aload is considerably small.

[0079]FIG. 5 is a circuit diagram showing a modified example of theswitching power supply shown in FIG. 1.

[0080]FIG. 6 is a circuit diagram showing a switching power supply thatis another preferred embodiment of the present invention.

[0081]FIG. 7 is a circuit diagram showing a conventional switching powersupply.

[0082]FIG. 8 is a timing chart showing the operation of the conventionalswitching power supply shown in FIG. 7.

[0083]FIG. 9 is a timing chart showing undershoot of the output voltageVout when an instruction for stopping the operation of the switchingpower supply is issued.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0084] Preferred embodiments of the present invention will be explainedin detail with reference to the drawings.

[0085]FIG. 1 is a circuit diagram showing a switching power supply thatis a preferred embodiment of the present invention.

[0086] As shown in FIG. 1, the switching power supply of this embodimentcan lower a DC (direct current) input voltage Vin supplied to a pair ofinput power terminals 41 and 42 to generate a DC output voltage Voutbetween a pair of output power terminals 43 and 44 and supply the DCoutput voltage Vout to a load. The switching power supply of thisembodiment is composed of a back converter circuit 50 connected to theinput power terminals 41 and 42, a half bridge converter circuit 60connected to the back converter circuit 50 for exciting a primary coilLp11 of a transformer T2, a back converter control circuit 71controlling the back converter circuit 50, a half bridge convertercontrol circuit 72 controlling the half bridge converter circuit 60, anauxiliary power supply circuit 80 for generating an operating voltageVcc for the back converter control circuit 71 and the half bridgeconverter control circuit 72 during normal operation, a first operatingvoltage generating circuit 90 for generating the operating voltage Vccduring the start of operation, a second operating voltage generatingcircuit 100 for generating the operating voltage Vcc during thetermination of operation, a rectifier 110 of a self-drive type locatedon the secondary side of the transformer T2, and a smoothing circuit 120connected between the rectifier 110 and the output power terminals 43and 44.

[0087] Further, an operation switch S11 is provided between the input DCpower source and the input power terminal 41. In order to activate theswitching power supply, the operation switch S11 must be changed to theON state from the outside. Although the operation switch S11 is not acomponent included in the switching power supply, it can be a componentincluded in the switching power supply.

[0088] The back converter circuit 50 includes main switches Q11 and Q12and a smoothing reactor Lb. As shown in FIG. 1, the main switch Q11 andthe smoothing reactor Lb are connected in series between the high-sideinput power terminal 41 and the half bridge circuit 60. The main switchQ12 is connected between the low-side input power terminal 42 and thenode of the main switch Q11 and the smoothing reactor Lb. The backconverter circuit 50 can lower the input voltage Vin supplied betweenthe input power terminals 41 and 42 to generate a DC internal voltage Vpbetween a pair of internal lines 45 and 46.

[0089] The half bridge converter circuit 60 includes main switches Q13and Q14 connected between the pair of internal lines 45 and 46 in seriesand primary side capacitors C11 and C12 connected in series between thepair of internal lines 45 and 46. As shown in FIG. 1, the primary coilLp11 of the transformer T2 is connected between a node of the mainswitches Q13 and Q14 and a node of the primary side capacitors C11 andC12.

[0090] The back converter control circuit 71 is a circuit forcontrolling the switching operation of the main switches Q11 and Q12included in the back converter circuit 50 so that the main switches Q11and Q12 are brought into the ON state in turn with intervention of apredetermined dead time under the control of the back converter controlcircuit 71. The back converter control circuit 71 controls the dutyfactor of the main switch Q11 based on the level of the output voltageVout. Specifically, the back converter control circuit 71 lowers theduty factor of the main switch Q11 when the output voltage Voutincreases relative to the desired voltage so as to decrease the electricpower supplied to the half bridge converter circuit 60 via the internallines 45 and 46, and raises the duty factor of the main switch Q11 whenthe output voltage Vout decreases relative to the desired voltage so asto increase the electric power supplied to the half bridge convertercircuit 60 via the internal lines 45 and 46. Thus, an internal voltageVp having a stabilized level which depends on the DC input voltage Vinand the duty factor of the main switch Q11 is supplied to the halfbridge converter circuit 60.

[0091] Because the back converter control circuit 71 belongs to theprimary side, the back converter control circuit 71 cannot receive theoutput voltage Vout directly. The back converter control circuit 71 istherefore supplied via an isolation circuit 130 with a voltage Vout′associated with the output voltage Vout. The minimum operating voltageof the back converter control circuit 71 will be explained later.

[0092] The half bridge converter control circuit 72 is a circuit forcontrolling the switching operation of the main switches Q13 and Q14included in the half bridge converter circuit 60 so that the mainswitches Q13 and Q14 are brought into the ON state in turn with a fixedduty factor. Thus, an output voltage Vout having a stabilized levelappears between the pair of output power terminals 43 and 44 whichdepends on the internal voltage Vp and the turn ratio of the transformerT2. The minimum operating voltage of the half bridge converter controlcircuit 72 will be also explained later.

[0093] The auxiliary power supply circuit 80 is a circuit for generatingthe operating voltage for the back converter control circuit 71 and thehalf bridge converter control circuit 72 during normal operation. Theauxiliary power supply circuit 80 is composed of a diode bridge circuitB, smoothing capacitors C13 and C14, a transistor Tr11, and resistorsR11 and R12. The diode bridge circuit B is a circuit for rectifying thevoltage appearing at an auxiliary coil Lp12 provided on the primary sideof the transformer T2. The voltage appearing between the output nodes ofthe diode bridge circuit B is smoothed by the smoothing capacitor C13 toproduce an auxiliary power voltage Vsub. The transistor Tr11 isconnected between the high-side output node of the diode bridge circuitB and a Vcc line. The resistor R11 is connected between the base andemitter electrodes of the transistor Tr11. The low-side output node ofthe diode bridge circuit B is directly connected to the internal line46. The auxiliary power supply circuit 80 having the above-describedstructure supplies an operating voltage having a predetermined level tothe Vcc line using the voltage appearing at the auxiliary coil Lp12 whenthe half bridge converter circuit 60 starts the switching operation. Theoperating voltage supplied to the Vcc line by the auxiliary power supplycircuit 80 is referred to as “Vcc1.”

[0094] The first operating voltage generating circuit 90 is a circuitfor generating the operating voltage for the back converter controlcircuit 71 and the half bridge converter control circuit 72 during thestart of operation. The first operating voltage generating circuit 90 iscomposed of a transistor Tr12, a resistor R13, a zener diode Z11, adiode D11, and an operation switch S12. As shown in FIG. 1, thetransistor Tr12 and the diode D11 are connected in series between thehigh-side input power terminal 41 and the Vcc line. The resistor R13 andthe zener diode Z11 are connected in series between the high-side inputpower terminal 41 and the low-side input power terminal 42. A node ofthe resistor R13 and the zener diode Z11 is connected to the baseelectrode of the transistor Tr12 so that a zener voltage V_(Z11) of thezener diode Z11 is applied to the base electrode of the transistor Tr12.Therefore, the voltage applied to the Vcc line when the transistor Tr12is brought into the ON state can be represented as:

[0095] V_(Z11)−2Vth

[0096] (where Vth represents both the voltage between the base andemitter electrodes of the transistor Tr12 and the forward voltage of thediode D11). The operating voltage supplied to the Vcc line by the firstoperating voltage generating circuit 90 is referred to as “Vcc2.”

[0097] In this embodiment, a zener diode Z11 is selected whose zenervoltage V_(Z11) is represented as:

[0098] Vcc1>Vcc2

[0099] Therefore, the first operating voltage generating circuit 90 isactivated only at the start of the switching power supply operation.After the half bridge converter circuit 60 starts the switchingoperation, the transistor Tr12 is brought into the OFF state so that thefirst operating voltage generating circuit 90 is not involved in theoperation of the switching power supply.

[0100] The operation switch S12 included in the first operating voltagegenerating circuit 90 is connected between opposite ends of the zenerdiode Z11. In order to activate the switching power supply, theoperation switch S12 must be turned OFF from the outside. That is, inorder to activate the switching power supply, the operation switch S11must be turned ON and the operation switch S12 must be turned OFF. Theoperation of the switching power supply can be terminated either byturning the operation switch S11 OFF state or by turning the operationswitch S12 ON.

[0101] The second operating voltage generating circuit 100 is a circuitfor generating the operating voltage for the back converter controlcircuit 71 and the half bridge converter control circuit 72 during thetermination of operation. The second operating voltage generatingcircuit 100 is composed of transistors Tr13 and Tr14, a zener diode Z12,and a diode D12. The transistor Tr13 and the diode D12 are connected inseries between the internal line 45 and the Vcc line. The transistorTr14 is connected between the internal line 45 and the base electrode ofthe transistor Tr13. The zener diode Z12 is connected between the baseelectrode of the transistor Tr13 and the internal line 46. The gate andsource electrodes of the transistor Tr14 are short-circuited so that thetransistor Tr14 acts as a constant current element. A resistor may beused instead of the transistor Tr14.

[0102] The zener voltage V_(Z12) of the zener diode Z12 is set lowerthan the zener voltage V_(Z11) of the zener diode Z11. Therefore, whenthe operation switch S11 is in the ON state and the operation switch S12is in the OFF state, a voltage lower than the threshold voltage of thetransistor Tr13 is applied between the base and emitter electrodesthereof because the base voltage of the transistor Tr12 becomes lowerthan the base voltage of the transistor Tr13, and then the transistorTr13 is kept in the OFF state. When the transistor Tr13 is in the OFFstate, the second operating voltage generating circuit 100 is notinvolved in the operation of the switching power supply.

[0103] When the transistor Tr13 is brought into the ON state, thevoltage applied to the Vcc line can be represented as:

[0104] V_(Z12)−2Vth

[0105] (where Vth represents both the voltage between the base andemitter electrodes of the transistor Tr13 and the forward voltage of thediode D12). The operating voltage supplied to the Vcc line by the secondoperating voltage generating circuit 100 is referred to as “Vcc3.”

[0106] Because the zener voltage V_(Z12) of the zener diode Z12 is lowerthan the zener voltage V_(Z11) of the zener diode Z11 as pointed outearlier, the relationship between Vcc2 and Vcc3 can be represented as:

[0107] Vcc2>Vcc3

[0108] Therefore, the second operating voltage generating circuit 100 isactivated only when the transistor Tr12 is in the OFF state owing to theoperation switch S11 turning OFF or the operation switch S12 turning ON.

[0109] In the switching power supply of this embodiment, the minimumoperating voltage of the back converter control circuit 71 is set lowerthan Vcc2 and equal to or greater than Vcc3, and the minimum operatingvoltage of the half bridge converter control circuit 72 is set lowerthan Vcc3. Therefore, the back converter control circuit 71 can drivethe main switches Q11 and Q12 when either the auxiliary power supplycircuit 80 or the first operating voltage generating circuit 90 is inthe active state, while the back converter control circuit 71 cannotdrive the main switches Q11 and Q12 when the second operating voltagegenerating circuit 100 is in the active state and neither the auxiliarypower supply circuit 80 nor the first operating voltage generatingcircuit 90 is in the active state. The half bridge converter controlcircuit 72 can drive the main switches Q13 and Q14 when any one of theauxiliary power supply circuit 80, the first operating voltagegenerating circuit 90, and the second operating voltage generatingcircuit 100 is in the active state.

[0110] The rectifier 110 is composed of rectifying switches Q15 and Q16and resistors R14 and R15. The rectifying switch Q15 is connectedbetween a secondary coil Ls11 of the transformer T2 and the low-sideoutput power terminal 44. The rectifying switch Q16 is connected betweena secondary coil Ls12 of the transformer T2 and the low-side outputpower terminal 44. The gate electrode of the rectifying switch Q15 isconnected to the secondary coil Ls12 and the gate electrode of therectifying switch Q16 is connected to the secondary coil Ls11. That is,the rectifier 110 is of a self-drive type. Further, the resistors R14and R15 are inserted between the gate and source electrodes of therectifying switches Q15 and Q16, respectively, so as to prevent the gateelectrodes thereof from being in the floating state.

[0111] The smoothing circuit 120 is composed of an output reactor Loutconnected between the rectified end of the rectifier 110 and thehigh-side output power terminal 43 and an output capacitor Coutconnected between the pair of output power terminals 43 and 44.

[0112] A load, which is not an element included in the switching powersupply, connected between the pair of output power terminals 43 and 44can be represented by a resistance component RLoad, capacitancecomponent CLoad, and reactance component LLoad.

[0113] Next, the operation of the switching power supply of thisembodiment will now be explained.

[0114]FIG. 2 is a timing chart showing the operation of the switchingpower supply of this embodiment.

[0115] In order to activate the switching power supply of thisembodiment, the operation switch S11 must be changed to the ON state andthe operation switch 512 must be changed to the OFF state from outside.When the operation switch 511 is changed to the ON state and theoperation switch 512 is changed to the OFF state, the level of the VCCline becomes Vcc2 because the transistor Tr12 in the first operatingvoltage generating circuit 90 turns ON.

[0116] Because of this, both the back converter control circuit 71 andthe half bridge converter control circuit 72 are activated.Specifically, the back converter control circuit 71 brings the mainswitches Q11 and Q12 into the ON state in turn with a certain dutyfactor based on the voltage Vout′ and the half bridge converter controlcircuit 72 brings the main switches Q13 and Q14 into the ON state inturn with a fixed duty factor.

[0117] Because of this, the polarity of the primary voltage V_(LP11) ofthe transformer T2 is alternately inversed and, synchronously with theoperation of the primary side, the secondary voltages appearing at thesecondary coils Ls11 and Ls12 of the transformer T2 are also alternatelyinversed, so that the rectifying switches Q15 and Q16 are alternatelybrought into ON state in turn. As a result, the secondary voltage ofalternately inversed polarity is rectified and the rectified voltage issmoothed by the smoothing circuit 120 so that a stabilized outputvoltage Vout is generated.

[0118]FIG. 3 is an enlarged timing chart showing a principal part of thetiming chart shown in FIG. 2.

[0119] As shown in FIG. 3, when the main switches Q13 and Q14 areswitched reciprocally under the control of the half bridge convertercontrol circuit 72, the frequency of the current I_(Lout) flowingthrough the output reactor Lout becomes twice the switching frequencyand the frequency of the current I_(Lp11) flowing through the primarycoil Lp11 of the transformer T2 becomes the same as the switchingfrequency. In the case where the direction of the current I_(Lout)flowing through the output reactor Lout is positive, the outputcapacitor Cout is charged; in the case where the direction of thecurrent I_(Lout) flowing through the output reactor Lout is negative,the output capacitor Cout is discharged.

[0120] During the period when the main switch Q13 is in the ON state,the primary side capacitor C11 is discharged while the direction of thecurrent I_(Lp11) flowing through the primary coil Lp11 of thetransformer T2 is positive and the primary side capacitor C11 is chargedwhile the direction of the current I_(LP11) flowing through the primarycoil Lp11 of the transformer T2 is negative. Although not shown in FIG.3, during the period when the main switch Q14 is in the ON state, theprimary side capacitor C12 is charged while the direction of the currentI_(Lp11) flowing through the primary coil Lp11 of the transformer T2 ispositive and the primary side capacitor C12 is discharged while thedirection of the current I_(Lp11) flowing through the primary coil Lp11of the transformer T2 is negative.

[0121] When the voltage produced by the switching operation of the halfbridge converter circuit 60 appears at the auxiliary coil Lp12 providedat the transformer T2, the auxiliary power supply circuit 80 suppliesthe operating voltage Vcc1 to the level of the Vcc line. Then, the firstoperating voltage generating circuit 90 is inactivated.

[0122] On the other hand, when the operation switch S12 is changed tothe ON state at a desired time, the transistor Tr12 turns OFF becauseits base voltage is lowered. When the transistor Tr12, turns OFF, thetransistor Tr11 also turns OFF because its base voltage is also lowered.

[0123] Because of this, the level of the Vcc line is lowered and thetransistor T13 then turns ON because the voltage between its base andemitter electrodes exceeds its threshold voltage. Then, the level of theVcc line becomes Vcc3, so that the operation of the back convertercontrol circuit 71 is terminated. That is, both of the main switches Q11and Q12 assume the OFF state. On the other hand, the main switches Q13and Q14 continue the switching operation because the minimum operatingvoltage of the half bridge converter control circuit 72 is set lowerthan Vcc3.

[0124] Therefore, the rectifying switches Q15 and Q16 also continue theswitching operation with a normal switching frequency and neither therectifying switch Q15 nor Q16 is kept in the ON state as in theconventional switching power supply.

[0125] As described in the foregoing, according to the switching powersupply of this embodiment, because the main switches Q13 and Q14continue the switching operation after an instruction for stopping theoperation of the switching power supply is issued by means of turning ONof the operation switch S12, energy stored in the output capacitor Coutand the capacitance component CLoad of the load is gradually consumed bythe resistance component RLoad of the load, the main switches Q13 andQ14, the rectifying switches Q15 and Q16, and so forth, so that theoutput voltage Vout is lowered. During this period, because the mainswitches Q13 and Q14 continue the switching operation with a normalswitching frequency, the output voltage Vout does not decrease whilefluctuating as in the conventional switching power supply but the outputvoltage Vout decreases substantially linearly.

[0126] Further, in the switching power supply of this embodiment, themain switches Q13 and Q14 continue the switching operation even afterthe operation switch S12 is changed to the ON state. Therefore, unlikein the conventional switching power supply, no flyback voltage arisesand thus the internal voltage Vp in the switching circuit is notincreased. The internal voltage Vp in the switching circuit decreaseslinearly as shown in FIG. 2. On the other hand, when the level of theVcc line lowers the minimum operating voltage of the half bridgeconverter control circuit 72, all of the switching operations areterminated. At this time, because the most of the energy stored in theoutput capacitor Cout and the capacitance component CLoad of the loadhas already been consumed, the output voltage Vout does not fluctuateover very long period as in the conventional switching power supply.

[0127] Furthermore, because the switching operation of the main switchesQ13 and Q14 after the operation switch S12 is changed to the ON state isthe same as the switching operation during normal operation, the currentflowing through the output reactor Lout is also the same as that ofnormal operation and no abnormal current flows.

[0128] Moreover, in the switching power supply of this embodiment,because the main switches Q13 and Q14 continue the switching operationafter the instruction for stopping the operation of the switching powersupply is issued by means of turning ON of the operation switch S12, aLCR serial circuit consisting of the rectifying switch Q15 (body diode),the secondary coil Ls11 of the transformer T2, the output reactor Lout,and the output capacitor Cout and another LCR serial circuit consistingof the rectifying switch Q16 (body diode), the secondary coil Ls12 ofthe transformer T2, the output reactor Lout, and the output capacitorCout oscillate when the formula (3) is satisfied: $\begin{matrix}{{{RLoad}\quad}^{2} < {4 \cdot \frac{LLoad}{{Cout} + {\left( \frac{N1}{N2} \right)^{2} \cdot \left( {{C11} + {C12}} \right)}}}} & (3)\end{matrix}$

[0129] where N1 represents the number of turns of the primary coil Lp11of the transformer T2 and N2 represents the number of turns of thesecondary coils Ls11 and Ls12 of the transformer T2.

[0130] As can be seen from the formula (3), according to thisembodiment, the LCR serial circuits resist oscillation because thecapacitances of the primary side capacitors C11 and C12 are added to theformula (1). Therefore, undershoot of the output voltage Vout can beprevented by utilizing the capacitances of the primary side capacitorsC11 and C12 without using any additional capacitor Cex.

[0131]FIG. 4 is a timing chart showing the operation of the switchingpower supply shown in FIG. 1 where the resistance component RLoad of theload is considerably small.

[0132] As shown in FIG. 4, in the case where the formula (3) is notsatisfied for the capacitances of the primary side capacitors C11 andC12, even if a voltage V_(LLoad) rises at the reactance component LLoadof the load, the output voltage Vout does not become negative becausethe LCR serial circuits do not satisfy the oscillating condition. Thatis, undershoot of the output voltage Vout can be prevented.

[0133] As explained above, according to the switching power supply ofthis embodiment, because by the operation switch S12 turning ON theoperation of the switching power supply can be terminated withoutoccurrence of the various problems which arise in the conventionalswitching power supply, the operation of the switching power supply ofthis embodiment can be started and terminated with the operation switchS11 in the ON state. Therefore, the switching power supply of thisembodiment is especially suitable in the case that the operation switchS11 is provided outside the switching power supply.

[0134] Further, in the switching power supply of this embodiment,because the first operating voltage generating circuit 90 is inactivatedduring normal operation while the operating voltage is supplied to theVcc line by the auxiliary power supply circuit 80, no electrical lossoccurs in the first operating voltage generating circuit 90 duringnormal operation.

[0135] Furthermore, because the switching power supply of thisembodiment performs a step-down of the input voltage Vin by two seriesconverter circuits, the back converter circuit 50 and the half bridgeconverter circuit 60, the electrical loss occurring in each convertercircuit can be decreased, so that allover conversion efficiency isenhanced.

[0136] Although FIGS. 2 and 4 show the case where the operation of theswitching power supply of this embodiment is terminated by turning theoperation switch S12 on, it can be also terminated by turning theoperation switch S11 off. Also in this case, the operation of theswitching power supply can be terminated without occurrence of thevarious problems which arise in the conventional switching power supply,similarly to the case of turning the operation switch 512 on.

[0137] In the switching power supply of this embodiment, although boththe collector electrode of the transistor Tr13 and the drain electrodeof the transistor Tr14, which are included in the second operatingvoltage generating circuit 100, are connected to the internal line 45,they can instead be connected to the high-side output node of the diodebridge circuit B included in the auxiliary power supply circuit 80, asshown in FIG. 5. The switching power supply shown in FIG. 5 can performalmost the same operation as the switching power supply shown in FIG. 1.

[0138] Next, another preferred embodiment of the present invention willbe explained.

[0139]FIG. 6 is a circuit diagram showing a switching power supply thatis another preferred embodiment of the present invention.

[0140] As shown in FIG. 6, the switching power supply of this embodimenthas the same structure as the switching power supply shown in FIG. 1except that an additional circuit 140 is added.

[0141] The additional circuit 140, which is composed of an additionalcapacitor Ca and an additional resistor Ra connected in series betweenthe internal lines 45 and 46, is used to prevent the LCR serial circuitsfrom oscillating. In this embodiment, when the formula (4) is satisfied,the LCR serial circuits oscillate so that undershoot arises.$\begin{matrix}{{{RLoad}\quad}^{2} < {4 \cdot \frac{LLoad}{{Cout} + {\left( \frac{N1}{N2} \right)^{2} \cdot \left( {{C11} + {C12}} \right)} + {2 \cdot \left( \frac{N1}{N2} \right)^{2} \cdot {Ca}}}}} & (4)\end{matrix}$

[0142] As can be seen from the formula (4), according to thisembodiment, the LCR serial circuits still more strongly resistoscillation because the capacitance of the additional capacitor Ca isadded to the formula (3). Therefore, the switching power supply of thisembodiment is suitable where undershoot of the output voltage Voutcannot be prevented by only utilizing the capacitances of the primaryside capacitors C11 and C12, i.e., when the formula (3) is satisfied. Itis worth noting that because the additional capacitor Ca and theadditional resistor Ra constitute a time constant circuit, theadditional circuit 140 does not affect the operation of the switchingpower supply during normal operation.

[0143] The additional circuit 140 can be added to the switching powersupply shown in FIG. 5.

[0144] The present invention has thus been shown and described withreference to specific embodiments. However, it should be noted that thepresent invention is in no way limited to the details of the describedarrangements but changes and modifications may be made without departingfrom the scope of the appended claims.

[0145] For example, in the above described embodiments, the desiredoperations can be realized by setting the zener voltage V_(Z12) of thezener diode Z12 lower than the zener voltage V_(Z11) of the zener diodeZ11. However, it is allowable for the zener voltage V_(Z11) of the zenerdiode Z11 and the zener voltage V_(Z12) of the zener diode Z12 to havethe same value or for the zener voltage V_(Z11) of the zener diode Z11to be lower than the zener voltage V_(Z12) of the zener diode Z12, solong as the base voltage of the transistor Tr12 is higher than the basevoltage of the transistor Tr13 when the operation switch S11 is in theON state while the operation switch S12 is in the OFF state.

[0146] Further, in the above described embodiments, the desiredoperations can be realized by setting the minimum operating voltage ofthe back converter control circuit 71 higher than that of the halfbridge converter control circuit 72. However, it is allowable for theseminimum operating voltages to have the same value or for the minimumoperating voltage of the back converter control circuit 71 to be lowerthan that of the half bridge converter control circuit 72, so long asthe back converter control circuit 71 is inactivated while the halfbridge converter control circuit 72 is kept in the active state when thesecond operating voltage generating circuit 100 is activated, by, forexample, adding one or more diodes in series between the Vcc line andthe Vcc input terminal of the back converter control circuit 71.

[0147] Furthermore, in the above described embodiments, the primary sidecircuit of the transformer T2 is composed of the back converter circuit50 and the half bridge converter circuit 60 connected in series;however, the primary side circuit of the transformer T2 is not limitedto this structure and other converter circuits can be used in series asthe primary side circuit of the transformer T2. For example, a boostconverter circuit or the like can be used instead of the back convertercircuit 50, and a forward converter circuit, full bridge convertercircuit, push-pull converter circuit or the like can be used instead ofthe half bridge converter circuit 60.

[0148] Further, in the above described embodiments, the rectifier 110 isof the self-drive type. However, because the problem that the outputvoltage Vout undershoots during the operation terminating arises whetherthe rectifier is a self-drive type or not, a rectifier of an ordinarytype using diodes or a rectifier of a synchronous type controlled by adriver circuit can be used instead of the rectifier 110 of theself-drive type.

[0149] Furthermore, in the above described embodiments, although theback converter control circuit 71 and the half bridge converter controlcircuit 72 belong to the primary side of the transformer T2, they canbelong to the secondary side of the transformer T2.

[0150] As described above, according to the switching power supply ofthe present invention, the operation of the switching power supply canbe terminated in such a manner that the output voltage Vout issubstantially linearly lowered without fluctuating or undershooting.Malfunction of the load can therefore be effectively avoided. Moreover,according to the switching power supply of the present invention, theinternal voltage Vp in the switching circuit on the primary side isprevented from gradually increasing when the instruction for stoppingthe operation of the switching power supply is issued. The electriccomponents used on the primary side are therefore effectively protectedfrom damage. Further, because it is not necessary to use componentshaving high withstand voltage, the cost of the switching power supplycan be lowered. Furthermore, because a large current does not flowthorough the output reactor Lout when the instruction is issued, thereliability of the switching power supply can be enhanced.

[0151] Therefore, the switching power supply of the present invention issuitable as switching power supply for supplying electric power to aload having a large capacitance component CLoad. Further, the switchingpower supply of the present invention is particularly suitable as aswitching power supply for supplying electric power to a load that tendsto frequently assume a light-load condition. Furthermore, the switchingpower supply of the present invention is also suitable as a switchingpower supply for supplying electric power to a load that requires a lowvoltage and a large current, such as a server computer. That is,according to the present invention, the switching power supply can lowerits output voltage Vout substantially linearly even if the instructionfor stopping the operation of the switching power supply is issued atheavy-load condition or light-load condition.

1. A switching power supply, comprising: a transformer having a primarycoil and a secondary coil; a switching circuit connected between aninput terminal and the primary coil of the transformer; a rectifierconnected to the secondary coil of the transformer; a control circuitcontrolling the switching circuit; and first and second operatingvoltage generating circuits each generating an operating voltage of thecontrol circuit; a first operating voltage generated by the firstoperating voltage generating circuit and a second operating voltagegenerated by the second operating voltage generating circuit havingdifferent values from each other.
 2. The switching power supply asclaimed in claim 1, wherein the first operating voltage generatingcircuit includes a first zener diode which determines a value of thefirst operating voltage and the second operating voltage generatingcircuit includes a second zener diode which determines a value of thesecond operating voltage, a zener voltage of the first zener diode and azener voltage of the second zener diode having different values fromeach other.
 3. The switching power supply as claimed in claim 2, whereinthe first operating voltage generating circuit further includes anoperation switch connected in parallel with the first zener diode. 4.The switching power supply as claimed in claim 1, wherein the switchingcircuit includes first and second converters connected in series betweenthe input terminal and the primary coil of the transformer.
 5. Theswitching power supply as claimed in claim 2, wherein the switchingcircuit includes first and second converters connected in series betweenthe input terminal and the primary coil of the transformer.
 6. Theswitching power supply as claimed in claim 5, wherein the controlcircuit includes a first converter control circuit controlling the firstconverter and a second converter control circuit controlling the secondconverter, the first operating voltage generating circuit supplying thefirst operating voltage to a power supply line commonly provided for thefirst and second converter control circuits, and the second operatingvoltage generating circuit supplying the second operating voltage to thepower supply line.
 7. The switching power supply as claimed in claim 6,wherein the first and second converter control circuits are enabled whenthe first operating voltage generating circuit is in an active state,and the first converter control circuit is disabled when the secondoperating voltage generating circuit is in an active state.
 8. Theswitching power supply as claimed in claim 6, wherein a minimumoperating voltage of the first converter control circuit and a minimumoperating voltage of the second converter control circuit are differentvalue from each other.
 9. The switching power supply as claimed in claim6, further comprising an auxiliary power supply circuit for supplying athird operating voltage to the power supply line using a voltageappearing at an auxiliary coil provided on the primary side of thetransformer.
 10. The switching power supply as claimed in claim 9,wherein the third operating voltage is higher than the first and secondoperating voltages.
 11. The switching power supply as claimed in claim4, wherein the first converter is selected from a back converter and aboost converter, and the first converter is selected from a half bridgeconverter, a forward converter, a full bridge converter, and a push-pullconverter.
 12. The switching power supply as claimed in claim 1, whereinthe rectifier is of a self-drive type.
 13. A switching power supply,comprising: a transformer having a primary coil and a secondary coil; aswitching circuit connected between an input terminal and the primarycoil of the transformer; a rectifier connected to the secondary coil ofthe transformer; a smoothing circuit located at a subsequent stage ofthe rectifier and including an output capacitor; a control circuitcontrolling the switching circuit; and means, responsive to aninstruction for stopping a switching operation, for supplying anoperating voltage to the control circuit using at least energy stored inthe output capacitor.
 14. The switching power supply as claimed in claim13, wherein the operating voltage supplied from the means is lower thanan operating voltage supplied to the control circuit during a normaloperation.
 15. The switching power supply as claimed in claim 13,wherein the rectifier is of a self-drive type.
 16. The switching powersupply as claimed in claim 14, wherein the rectifier is of a self-drivetype.
 17. A switching power supply, comprising: a transformer having aprimary coil and a secondary coil; first and second converters connectedin series between an input terminal and the primary coil of thetransformer; a rectifier connected to the secondary coil of thetransformer; a control circuit controlling the first and secondconverters; and means, responsive to an instruction for stopping aswitching operation, for stopping operations of the first and secondconverters in this order.
 18. The switching power supply as claimed inclaim 17, wherein the means causes the operation of the second converterto continue using energy supplied from a secondary side of thetransformer during a period from a time when the instruction is issuedto a time of the operation for stopping the second converter.
 19. Theswitching power supply as claimed in claim 17, wherein the rectifier isof a self-drive type.
 20. The switching power supply as claimed in claim18, wherein the rectifier is of a self-drive type.